Well-known transparent film supports used for silver halide photographic projection film elements or photographic film sheets to be examined as for e.g. color print film applications, can be e.g. cellulose triacetate, a polyester like a polyalkylene terephthalate or naphthalate e.g. polyethylene terephthalate or polyethylene napthalate, or polycarbonate.
As is further well-known, marking of such elements, before or after exposure to heat or radiation can be performed by means of mechanical or chemical methods, by heat or radiation, wherein said technique of radiation may be performed by means of a laser beam, travelling over the zone of film that should be marked.
Marks added to a photographic film element may be e.g. graphic elements, characters, bar codes or text.
Conventional chemical methods often make use of a technique wherein the surface of the film is initially coated with a topcoat afterlayer, which is sensitive or made sensitive to marking techniques by radiation, heat, etc. and wherein after etching the marked zones, which behave differently from the non-marked ones, a washing process cleans up the film element.
As is clear from this presentation huge amounts of chemicals are required and as a consequence this inevitably causes environmental load. Moreover chemical methods are rather complicated, as more steps are required, and as a consequence the said chemical methods are penalised by the slowness of the treatment, the need to establish artwork and the need to use plates of very high quality in order to get sharp marks. Further aspects should not be neglected as there are: its sensitivity to the accuracy with which various parameters should be adjusted such as the relief and the pressure of the said plates, the problems of choosing reagents and of irregularities in adhesion and in consistency of the protective layer, the need to take into account the sensitivity and the defects of the element to be marked, the slowness and the instability of the mechanical contact between the film element and the plate.
To summarize: chemical marking methods are much more complicated than e.g. laser engraving methods. By means of a laser beam of a high energy laser, travelling along a determined path corresponding to the inscriptions to be formed on the print film element, the silver halide photographic emulsion present in the layer(s) coated onto the support, becomes ablated locally.
In the particular application of laser subtitling methods of color print films, the quality of the result depends on the nature of the emulsion(s) and on the background density of the images on which the subtitles are to be etched. Working parameters, in particular the power of the laser beam radiation applied to the film and the displacement speed of the laser beam, are determining the sharpness of the subtitles to a large extent.
In said laser subtitling methods the power, the exposure time and the wavelength are selected so as to destroy the photographic image forming emulsion elements completely at points where the laser beam strikes the film. Due to the non-uniform release of gelatinous residues, gelatin present as a main protective hydrophilic colloid in the coated photographic layers, or to the damage of the support and as a result thereof leaving dark spots and coloured spots, an unquiet and unpleasant enlarged view is projected on the screen in the cinema.
A method and apparatus for subtitling color multilayer motion pictures or photographic plates by means of continuous or broken lines with a laser have been described in EP-A 0 201 391.
Another method and apparatus makes use of a mask to produce subtitles and has been described in EP-A 0 282 611. Further EP-A 0 464 270 describes the application of a protective layer over at least portions of a film or tape, whereupon characters should be written by means of a laser beam.
An advanced environmental friendly system inscribes textures from a PC through the support side of the film by means of a laser and has been described in DE 4 109 545: a specific advantage herein is that no chemicals are used and that there is no waste water at the end of the subtitling cycle.
An optimised method of subtitling motion pictures by laser beam travelling, directly engraving a film element, has recently been disclosed in U.S. Pat. No. 5,367,348. Said method essentially comprises the steps of travelling the laser beam over the film at a speed of displacement V lying in the range from about 1 to about 200 cm/s with a power of the laser P lying in the range of about 100 mW to about 20 W, in order to have a ratio V/P lying in the range from 10 to 30, followed by a washing step in order to remove particles that have been heated and disclocated from the zones of the film that have previously been marked by the low power laser beam. As has been disclosed in the said US-Patent the method is well adapted to subtitle motion picture films on a support constituted by a cellulose derivative such as cellulose triacetate, but is equally applicable to film supports based on a thermoplastic polymer material, such as polyester. However if use is made of a transparent polyester support such as e.g. polyethylene terephthalate, it is not possible to obtain zones without undesired spots, resulting in disturbing optical failures. Moreover lowering of the power of the laser results in grayish or slightly yellow coloured zones.